The present invention relates to a method and apparatus for detecting a focus error which are used when information is recorded and/or reproduced by irradiating a light beam onto an optical recording medium and, more particularly, to a focus error detection method and apparatus in which a light beam having an astigmatism is focused onto the medium surface and a focus error signal is detected by using the reflected light. The invention also relates to an optical information processing apparatus to record and/or reproduce information onto/from an optical recording medium such as an optical disk by using such a focus error detecting apparatus.
Most of focus error detection systems which are used in conventional optical disk devices or the like use a principle such that a shape of a reflected light beam from the disk changes due to a focus error. The reflected light beam is detected by a multi-divided photo-detector and an unbalance of outputs of detector elements of the multi-divided photo-detector which is caused due to a change in shape of the reflected light beam on the multi divided photo-detector due to the focus error is used as a focus error detection signal.
As such a kind of focus error detection system, for instance, there has been known an astigmatism system disclosed in, e.g., U.S. Pat. No. 4,293,944. That is, when an astigmatism is given from an astigmatism device such as a cylindrical lens or the like to the reflected light beam from a disk, two astigmatic foci or caustic surfaces which are perpendicular to each other are formed at separated positions and the light beam becomes a circle at the position of the circle of least confusion almost at the center between the astigmatic foci. Therefore, a four-divided photo-detector is arranged at the circle of least confusion position to detect the reflected light from the disk, outputs of the photo-detector elements of the four-divided photo-detector are added for respective two pairs of photo-detector elements arranged at diagonal positions, and the difference between two addition signals is calculated, so that a focus error detection signal is derived.
When setting an objective point for focusing, the point at which the data signal or the like becomes maximum or the point at which the reflected light amount which is returned to a light source becomes maximum is detected by another measuring system different from the focusing system, thereby setting the zero point (objective point for focusing) of the focus error detection signal.
In the above astigmatism system, for instance, there is a case such that when a light spot passes through a data pit or the like on the disk surface, to the focus error detection signal is nonlinear with respect to focus deviations so that the auto focusing control system oscillates. This is because when the light spot passes through a data pit or the like, a space distribution of light intensities of the light beam reflected from the disk changes, so that an unbalance is caused among the outputs of the photo-detector elements of the multi-divided photo-detector.
On the other hand, there is a problem such that when the attaching positions of the optical parts are deviated due to a temperature change or the like, the position of the light beam on the multi-divided photo-detector surface is deviated, so that an unbalance occurs among the outputs of the photo-detector elements of the multi-divided photo-detector and the zero point position of a focus error detection signal is deviated from a focusing objective point.